Chris Borland, Fearing for Health, Retires From the 49ers. At 24.

It is an off-season like no other in the National Football League. Young players, with many games and millions of dollars potentially ahead of them, are walking away from the country’s most popular sport.

Linebacker Chris Borland of the San Francisco 49ers, one of the top rookies in the N.F.L. last season, is the latest case, and perhaps the most noteworthy. He said Monday that he was retiring because of concerns about his safety, and his decision may have ripple effects well beyond the professional ranks.

“Somebody said we’re at the beginning of the beginning, and that might be true,” Jeff Borland, Chris’s father, said Tuesday in a telephone interview regarding whether his son’s decision would influence parents of young football players.

During the preseason last year, Chris Borland sent a letter to his parents in Ohio. He thanked them for their support and said he was embarking on what was likely to be his final season playing football.

“We readily accepted it,” Jeff Borland said. “It was a relief that after one more season, he would no longer be taking the physical abuse. He kept his unpublished word that he gave the team everything he had. I was very proud of that.”

Borland’s decision, which was first reported by ESPN, came just days after Patrick Willis, 30, a seven-time All-Pro linebacker also with the 49ers, announced that he would retire rather than risk further injury.

At least three other prominent players have followed suit.

But Borland, 24, who played at the University of Wisconsin, is the youngest and most promising of the players who have left the N.F.L. this off-season.

“From what I’ve researched and what I’ve experienced, I don’t think it’s worth the risk,” Borland told ESPN, referring to the multiple concussions he had sustained.

Cornerback Cortland Finnegan, 31, who last played for the Miami Dolphins; quarterback Jake Locker, 26, of the Tennessee Titans; and linebacker Jason Worilds, 27, of the Pittsburgh Steelers have retired this off-season for a variety of reasons.

Many veteran players have cited the physical toll of football as they have stepped away, but rarely have so many young players left the league with so much of their careers ahead of them. Worilds, who was paid $9.75 million by the Steelers in 2014, had been expected to sign a large contract with another team as a free agent.

Borland stands out because of how explicit he was regarding his concerns about head injuries. As evidence has mounted in recent years linking blows sustained on the field with long-term cognitive disability, the league has scrambled to find an appropriate response.

For years, N.F.L. officials disputed the work of independent researchers and refused to acknowledge any connection between head trauma sustained on the field and long-term cognitive problems. More recently, the league has changed rules to try to mitigate some of the most jarring blows to the head and assigned neurologists to the sideline during games.

The N.F.L. said in a statement that “playing any sport is a personal decision.” The statement, from Jeff Miller, the league’s senior vice president for health and safety policy, added: “We are seeing a growing culture of safety. Everyone involved in the game knows that there is more work to do, and player safety will continue to be our top priority.”

Borland, a native of Kettering, Ohio, who studied history and was ranked sixth in career tackles at Wisconsin, had a stellar first year after being picked in the third round of the 2014 N.F.L. draft. He was honored twice as a rookie of the week and was the defensive rookie of the month in November. He earned the league-minimum $420,000 and a bonus of $154,000, according to the website Over the Cap.

Borland’s father said he knew the dangers his son faced every Sunday. He kept his sons from playing football until high school because he thought youth coaches did not teach the game properly. And having watched his son from the sideline in games and practices in high school and college, he knew how much bigger and faster the players were in the N.F.L., and how the risk for injury was greater.

“There are players trying to hang on to a job and have to do anything they can,” Jeff Borland said.

Despite his success his rookie year, Chris Borland was unwilling to play another season, his father said, because he was concerned that it would become a trap — he would continue risking injury in pursuit of a paycheck.

“For him to have reached the end of the season after having the opportunity and success he had with it, to have not changed his mind, I have to give him credit,” Jeff Borland said. “It was not a money decision.”

Several current and retired N.F.L. players expressed support for Borland on social media. “I loved Chris Borland’s game but I can’t fault him for calling it quits,” Chris Long, a defensive lineman on the Rams, wrote on Twitter. “His concerns are real. Still, it takes a man to do the logical.”

Donte Stallworth, who last played in 2012 after a 10-year career with six teams, saw Borland’s retirement as a result of growing awareness of the potential long-term cognitive damage facing players. “Players today are more concerned now than ever before regarding brain trauma and health issues,” he posted on Twitter. “It’s scary!”

The average career in the N.F.L. is about three and a half years, but that figure includes players who try out in training camp but fail to make a team, as well as those who last a dozen or more years.

The N.F.L. and the N.F.L. Players Association do not keep precise figures on players’ reasons for retiring. Some are cut by teams and are not re-signed, while others are injured and cannot play.

The N.F.L. said that players who were drafted in the first round played for an average of almost nine years, while players who made it to an opening-day roster averaged close to six years in the N.F.L.

The N.F.L., with its opportunity for million-dollar paydays, will remain a draw for players across the country, particularly in states like Florida and Texas where football remains immensely popular and where some parents think the game is one of the best ways to secure a college scholarship.

But the increasing evidence of links between repeated head trauma and long-term cognitive problems has persuaded more parents to steer their children into sports like baseball, basketball and soccer. The decision by a highly regarded player like Borland to leave the N.F.L. while still young might accelerate that trend.

A recent Bloomberg Politics poll found that half of Americans did not want their sons playing football. A separate survey by Robert Morris University showed that nearly half of those polled said that boys should not be allowed to play tackle football until they reached high school.

While there is constant turnover in the N.F.L., Borland’s retirement may not prompt a string of other young, talented professional players to retire simply because of concerns about brain trauma. Robert Boland, who represented N.F.L. players for 12 years and now teaches sports management at New York University, said players retired for many reasons, including injuries, other ambitions and a loss of passion for the game.

Competition to join an N.F.L. roster is so intense that the 49ers are unlikely to have trouble finding candidates to fill Borland’s spot.

But his retirement could send a chilling signal to parents who are debating whether to let their children play tackle football. If they see his retirement as evidence that talented players view the game as too risky, it could further erode participation in youth football leagues and ultimately shrink the pool of potential N.F.L. talent.

“Some parents will say it’s proof positive that football is too dangerous,” said Boland, the teacher. “It’s a nightmare for the N.F.L. and colleges because it shrinks the reach of the game. It could lead the image of the game down the path where the league or owners don’t want it to go.”

The increasing specialization in youth sports may also be having an effect. As more children concentrate on a single sport all year, they are becoming more prone to injury and burnout, said Brandon Huffman, the director of recruiting for Scout, a website that tracks football players from youth leagues to the N.F.L.

“The arms race for college scholarships has led to a real physical strain,” he said. “You’ve got parents who are putting their kids in youth football at 6 or 7, and they’re trying to stretch the age limits. When kids get to high school and college, they lose their passion.”

Overspecialization did not appear to be the case with Borland, who told ESPN that he had received diagnoses of two concussions before arriving at Wisconsin. One was sustained while he played soccer in eighth grade and the other while he played football in high school.

“While unexpected, we certainly respect Chris’s decision,” Trent Baalke, the 49ers’ general manager, said in a statement Tuesday. “From speaking with Chris, it was evident that he had put a great deal of thought into this decision.”